This invention relates generally to floor and roof reinforcing support structures for buildings, and more particularly to such a floor or roof structure utilizing a plurality of joists as integral parts of roof or floor sections such as a poured concrete slab composite construction.
Over the past several years the need for stronger, lighter, less costly, and more durable roof and floor structures along with the need for more uniform materials has led to an ever increasing interest in steel joists and reinforcing members for floors and roofs. While various built-up, sheet metal and open truss shapes have been tried with various levels of success, few have met the criteria of manufacturing simplicity, flexibility for piping and electrical access, as well as ease in installation.
The flooring and roofing systems of buildings are complex integrated systems of components that must act together in a reliable and cost-effective manner during transportation, installation, and in service, where modifications are sometimes common.
One common approach taken by the building products industry to address these diverse needs is that of welded-member truss sections. These trusses are usually welded combinations of steel L-angle and round bar components. While these steel trusses can provide fairly good access for electrical and tubing routing needs, they are labor-intensive and require often-complex quality control measures associated with the weldment that are an integral part of their manufacture. As a result, they can be costly for a builder to specify. In addition, the stock must often be ordered to xe2x80x9cexact length,xe2x80x9d since any required modifications at the job site may be difficult and involved. This has led to quite restricted use of these trusses, especially in the residential building marketplace for truss lengths generally under 20 feet.
Still other members have consisted of thin sheet metal webs reinforced by angles as top and bottom chord or flange members. However, these have not gained wide acceptance for various reasons including the following. First, the top and bottom angle members are usually thicker than the web member, making welding without excessive imperfections in the thin sheet a difficult process. In addition, the welded portions are located in relatively high stress regions, and may be weakened by corrosion, since welding usually removes any pre-existing corrosion protection coatings. Furthermore, the nesting required for efficient stacking and transportation is an especially difficult problem, since these sections are easily damaged during transport and installation.
Still other approaches have included various wood I-beam built-up trusses where the top and bottom chords are glued or mechanically fastened to a web member. While these trusses are quite flexible and simple to install for intermediate applications, they are of limited utility for longer spans. Furthermore, they do not lend themselves for use in composite flooring systems because they lack the strength and rigidity to be integrated adequately with concrete aggregates.
While thin sheet metal hat-shaped Z-shaped, and C-channel cross-sections have been considered, these sections have some inherent disadvantages. One of these disadvantages is that these truss members or joists have a xe2x80x9cblade edge.xe2x80x9d This edge is very susceptible to imperfections in the sheet metal along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the truss or joist can initiate. A more detailed description of this failure initiation follows.
Even the most perfect, smooth edge of the conventional sheet metal truss member or joist will experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads. Thus, initiation of an edge xe2x80x9cbulgexe2x80x9d or xe2x80x9ccrimpxe2x80x9d on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or xe2x80x9cpropagatexe2x80x9d easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the truss member material itself
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the wind load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional truss member has the effect of enhancing an already established process of failure initiation.
Second, these truss members or joists, when manufactured out of relatively thin sheet metal are more susceptible to buckling due to the reduced thickness. Buckling is an instability in a part of the truss member associated with local compressive or shear stresses. Buckling can precipitate section failure of the truss member. For example, in a Z-section truss member with edge lips on the flange edges, when the top and bottom flanges are non-uniformly stressed, the result can be a kinking of the edge in the form of a crimp or buckle. This crimping can lead to complete failure of the section.
Finally, some thinner conventional truss members can experience xe2x80x9crollingxe2x80x9d when placed under load. Rolling is when the shear stresses within the truss member results in a net torque about the centroid. of the thin walled cross-section thus causing the cross-section to twist possibly making the truss member unstable. Some manufacturers have increased the cross-sectional length of the flanges of the conventional C-channel stiffener or joist member trying to solve the rolling problem but were met with only marginal improvement. This is because the increased flange length had the simultaneous effect of increasing the distance from the centroid to the shear center of the channel. Additionally, increasing the cross-sectional flange length caused difficulty in accessing the fasteners used in mounting the C-channel to the rest of the integrated structure.
Because of diverse market requirements, the need for a simple, scalable, and reliable truss member, and the problem of joining relatively thick sections to sections relatively less thick, there is a need within the industry today for a versatile new lightweight/lower cost truss or joist configuration that can address all of the above-mentioned drawbacks and short comings of the present state of the art, is suitable for use with substantially all standardized building methods, and can be made on a cost-effective basis.
The present invention alleviates and overcomes the above-mentioned problems and shortcomings of the present state of the art through a novel lightweight/lower cost joist member. The novelty and uniqueness of this invention is that it: 1) is made of thinner material to reduce the in-plane stresses found in the fastener or joint area when it is integrated with other structures, 2) resists deflection adequately to meet stringent building code requirements, 3) is resistant to buckling and rolling, 4) effectively addresses edge stress concentrations by modifying the blade edge to an area of relatively low stress, and 5) can be manufactured cost effectively by using conventional manufacturing methods such as roll forming.
This novel invention may be described as a substantially reconfigured or stabilized J-section sheet metal truss having a mounting or integrating flange. It should be noted here that due to their extreme susceptibility to rolling, conventional J-section sheet metal joist members are seldom used in buildings. The unexpectedly strong synergisms of the unique characteristics found in the present stabilized J-section truss not only address the above problems, but simultaneously obtain material savings. More particularly the synergisms may be described as follows.
The instant invention has substantially redistributed material at critical locations as compared with conventional metal truss configurations. This material redistribution has the effect of altering considerably the behavior of the truss as compared with conventional J-sections and other truss configurations. The material redistribution required to accomplish these collaborative effects is accomplished by having specifically placed free edge portions, which are turned to define tubular beads or curls along the free edges. Moreover it is not just the presence of the tubular bead or curl that enables the substantial level of synergism, but the discovery of specific ratios of curl diameter to other truss member dimensions that maximize these synergisms even to the extent of obtaining significant weight or cost savings.
Two sets of significant synergisms combine to make the present invention successful. The first set of synergisms is directly related to the ratio of the diameter of the curl to the truss section flange length and web length. Each tubular bead has a cross-sectional dimension which when combined in specific ratios with other truss member dimensions substantially maximizes the moment of inertia of the overall section about the horizontal and vertical axes with a minimal use of material. Moreover, the tubular bead size specified by these same ratios has the effect of altering the characteristic failure mode normally associated with the free edge stress concentration for conventional steel truss members as described above. Finally, the cross-sectional dimension of the tubular beads of the stabilized J-section truss member make this novel truss member less sensitive to edge imperfections and damage because the blade edge has now been placed in a position of relatively benign stress levels so that imperfections or damage to the tube or edge region must now be on the order of size of the diameter of the curl in order to have significant detrimental effect on the truss member section.
Having established the above ratios, a second set of synergisms was discovered by directly combining the above with specific ratios of the truss""s cross-sectional web dimension to crosssectional flange dimension. The compounding effect of the first set of synergisms with this additional set of ratios makes the stabilized J-section truss member or joist more resistant to rolling and buckling and thus avoids the problems that plague deeper conventional truss members using thinner gauge material. Additionally, these compounding synergisms make this truss member unique in that stresses are now more evenly distributed in the flanges thus making the truss member more stable and less sensitive to dimensional imperfections. Because of these cooperative effects, the stabilized J-section truss member demonstrates its uniqueness and efficiency in using thinner gauge material to reduce in-plane stresses found in the fastener or joint area, thus allowing the composite floor or roof structure including concrete and steel joist members to work together as a cohesive system instead of as individual components.
When compared to conventional truss members on the market today, the stabilized J-section truss member uses substantially thinner material while obtaining better resistance to structural loads. Thus even though additional slit width (width of the sheet of material from which the truss is made) is required to reposition needed material, the use of thinner gauge material more than offsets the additional slit width, bringing overall material savings as high as 25% in many instances. This innovation in system configuration also represents a substantial cost savings for the manufacturer, since material cost is a substantial portion of total manufacturing costs for building hardware. Thus, this unique and novel truss member is very cost effective.
For manufacturing process cost efficiency, the tubular bead is preferably an open-section bead, meaning that the sheet metal is formed in an almost complete bend or curl, but the curl need not be closed at its outer edge, such as by welding. A closed section tubular bead would work equally well, at a slightly higher manufacturing cost.
This edge feature is discussed in more detail in the following paragraph. The joint or integration section curl and the trough curl are tubular features, preferably open-sections, that are made by shaping the free edges or edge marginal portions of the truss cross-sections into an elliptical, preferably circular (for manufacturing simplicity), cross-sectional shape. As used herein, a circular cross-section is considered to be a special case of an elliptical cross-section. The term xe2x80x9ccharacteristic diameterxe2x80x9d refers to a constant diameter in the case of a circle, while other elliptical shapes will have major and minor axes or diameters, with the major axis or diameter being the xe2x80x9ccharacteristic diameterxe2x80x9d. Even though some configurations of a slightly non-circular elliptical shape may be more desirable in some applications, the circular cross-section is generally preferable, because it is simpler to manufacture, while still achieving the desired benefits to a significant degree.
It is important to contrast the edge curl approach against other possible edge treatment approaches by noting that the dimensional order of size effect related to imperfections or damages described above for the curl can not be achieved by simply folding the edge over, either once or multiple times, because in this case the characteristic dimension will be defined by the fold edge diameter and not by the length of overlap of the fold. This is because the overlap direction is transverse to the edge and quickly moves out of the peak stress region, and because the edge fold diameter defines the maximum distance over which the edge stresses may be effectively spread.
The elliptical or circular open-section tubular shape or xe2x80x9cedge curlxe2x80x9d is contrasted to tubular sections of rectangular cross-sectional shapes, including folded edges, and to open-section tubular shapes of softened corner rectangular cross-sectional shapes in that in general, the characteristic diameter will be defined in each of these other cases by the fold diameter or by the softened corner diameter nearest to the truss member edge, as opposed to the overall diameter of the edge curl section. It may be noted that in this context a rectangular cross-section with very softened corners is in effect an imperfect ellipse or circle. In some instances, quasi-elliptical or quasi-circular crosssections, imperfect ellipses, and imperfect circles, such as in the form of rectangular cross-sections with very softened comers may function adequately, but may also be more difficult to manufacture and will be less effective than a generally circular curl.
The resulting synergistic effect of the stabilized J-section truss member""s material efficiency in obtaining the desired bending moment of inertia, the alteration of the characteristic failure mode, the reduction in sensitivity to edge imperfections and damage, resistance to buckling and rolling as well as the ability to spread stresses more uniformly, has the same degree of compounding advantage as some conventional truss or stiffener""s compounding disadvantage of low resistance to buckling and rolling combined with sensitivity to relatively small edge or dimensional imperfections. Accordingly, it can now be appreciated by those versed in this art, that the novel stabilized J-section truss members of the instant invention provide a solution to the problems that the building truss member art that has sought in order to overcome the shortcomings associated with conventional sheet metal truss configurations available hitherto. In fact, the present truss member is even competitive with traditionally highly competitive open-section truss members that are composed primarily of welded rods and L-angle members. In this case the competitive edge obtained for shorter spans includes both weight and manufacturing cost, while for greater spans it consists primarily of significant manufacturing cost savings. In summary, the stabilized J-section truss of the present invention has mounting or integrating flanges that may be uniquely designed to be compatible with substantially all standard building member interfaces, thereby significantly reducing the number of truss member types that manufacturers must carry in their inventories and package. This permits a great variety of building needs and requirements to be met, and does so without major modification of other structural components.
The following description of the present invention may incorporate dimensions which are representative of the dimensions which will be appropriate for most commonly found building structures. Recitation of these dimensions is not intended to be limiting, except to the extent that the dimensions reflect relative ratios between the sizes of various elements of the invention, as will be explained where appropriate.
It is a object of this invention to provide joist members for a floor or roof structure for buildings with the joist member being formed of minimal steel material while providing necessary strength for the floor or roof structure.
It is a further object of this invention to provide a composite floor or roof structure formed of reinforced cement and integral joint members of a thin gauge material having upper flanges embedded in the concrete.
It is another object of this invention to provide integral one piece joist members in which upper and lower flanges of the joist members have free edges with tubular beads or curls formed on the free edges to stiffen the flanges. Other objects, features, and advantages of the invention will be apparent from the following specification and drawings.